Pd-Catalyzed CO/Vinyl Arene Copolymerization: when the Stereochemistry is Controlled by the Comonomer

Abstract

The control of the stereochemistry of macromolecules is a very important goal, and coordination-insertion polymerization is superior with respect to the other polymerization methods for its achievement. In this contribution, we focus on Pd(II) homogeneous catalysts for the stereocontrolled synthesis of CO/vinyl arene polyketones. We developed a library of aldo-and keto-iminopyridine ligands N-N′ that feature an αor β-naphthyl or anthracenyl moiety on the imino nitrogen atom (N imm ). With such ligands, the Pd(II) monocationic complexes [Pd(CH 3 )(CH 3 CN)-(N-N′)][PF 6 ] were synthesized. NMR spectroscopy shows that in solution, each complex exists as an equilibrium mixture of cis and trans stereoisomers, the latter having the CH 3 ligand opposite to the Pd−N imm bond. The isomeric population depends on the N-N′ ligand: an almost 1:1 ratio is found for the ketimine complexes, whereas those with the aldimines show a preference for the trans geometry. These complexes generate very efficient catalysts for the CO/vinyl arene copolymerization. Catalyst performances depend both on the nature of N-N′ and of the vinyl arene comonomer. The ketimine-based catalysts are more stable and more productive than the aldimine counterpart, leading to prevailingly syndiotactic macromolecules of high M w (up to 280 kDa). The aldimine derivatives produce copolymers with isotactic and syndiotactic stereoblocks of different lengths depending on the vinyl arene. The effect of the prochiral monomer on the copolymer tacticity is steric in nature as demonstrated by the stereochemistry of the obtained CO/4-fluorostyrene polyketone, whose synthesis is reported here for the first time. As a conclusion, we have now demonstrated that when catalysts with nonsymmetric ancillary ligands are used, and stereoisomers are present, the stereochemistry of the copolymerization is driven by both the catalyst isomeric distribution and the prochiral comonomer.

Alternating copolymerization of carbon monoxide and vinyl arenes using [N,N] bidentate palladium catalysts

“…Besides, aniline moieties also affected CO/vinyl arene copolymerization. Palladium catalysts with the β ‐naphthyl groups showed higher productivities than the corresponding α ‐derivatives 125 . Because the β ‐naphthyl moiety was more distant from the palladium center than the corresponding α ‐counterpart, a lower steric congestion was created in the coordination plane with positive effects on both the growing chain and the incoming comonomer.…”

“…Recently, the steric effect of 4‐substituent was gradually discovered. Milani found that the stereochemistry of polyketones produced by aldimine‐based pyridine‐imine palladium catalysts was dependent on 4‐substituent of the vinyl arene comonomer 123–125 . Therefore, steric effect of 4‐substituent of comonomer was proposed.…”

“…The ketimine catalysts with the 1:1 ratio of cis ‐ and trans ‐ isomers produced prevailingly syndiotactic copolymers but the trans‐ aldimine catalysts produced copolymers with isotactic and syndiotactic stereoblocks of different lengths depending on the comonomer 124 . The stereochemistry was dependent on the steric effect of the comonomer 124,125 …”

“…Milani reported aldo-(R═H) and keto-iminopyridine (R═Me) homogeneous Pd(II) catalysts with different aniline moieties for CO/vinyl arene copolymerizations. [123][124][125] It was found that the ketimine catalysts (R═Me, Ph) (PI-2a ~PI-2f, PI-3a ~PI-3b) were more active than the corresponding aldimine catalysts (R═H) (PI-1a ~PI-1g) with an order of magnitude, no matter what kind of Ar groups at the imine moiety. This result was ascribed to the strong coordination capability of the ketimine ligands and higher catalyst stability.…”

Modulation of N^N′-bidentate chelating pyridyl–pyridylidene amide ligands offers mechanistic insights into Pd-catalysed ethylene/methyl acrylate copolymerisation

“…21,22 Another class of versatile unsymmetrical ligands is constituted by iminopyridines, which have found successful application in Pd-catalysed CO/vinyl arene copolymerisation. [23][24][25] As an extension of this approach, we have recently demonstrated that Pd(II) complexes bearing bidentate pyridylidenamide (PYA)-derived ligands rapidly catalyse the dimerisation of ethylene and give a high degree of methyl acrylate (MA) incorporation, though rapid chain transfer yielded only low molecular weight products. 26 These PYA ligands have great potential for tailored catalysis since they are readily functionalisable and electronically flexible due to their ability to exist in configurations with varying contributions from zwitterionic and neutral resonance forms (Scheme 1).…”